Wafer-to-wafer bonding techniques are used to create substrates with varying layer-to-layer characteristics for a variety of different applications. For example, wafer-to-wafer bonding, i.e., wafer bonding, can allow the formation of device layer stacks that otherwise cannot be grown or deposited with conventional thin film methods with the same material qualities required for high device performance
Wafer bonding techniques typically employ at least two wafer substrates, referred to as a handle wafer and a donor wafer, as starting materials. Wafer bonding is generally done in three basic operations: (1) cleaning and surface activation of the handle wafer and the donor wafer prior to bonding; (2) bringing into contact the treated and cleaned surfaces of the handle wafer and the donor wafer; and (3) post-anneal processing to seal the bond.
In some applications, a substrate of varying layers having different crystal orientations is desired. “Crystal orientation” refers to the crystal lattice structure of materials used in the fabrication of semiconductor substrates. Crystal orientation planes of silicon are typically represented as (100), (110) and (111) and are representatively shown in FIG. 1. Monocrystalline silicon is an anisotropic material, meaning that the properties of monocrystalline silicon change depending on the direction from which they are measured within the crystal lattice of silicon. This may be explained by the different atomic densities within each of the (100), (110), and (111) crystal planes that are illustrated in FIG. 1. The atomic densities of the (100) crystal plane, the (110) crystal plane, and the (111) crystal plane are illustrated in FIG. 1. Examples of properties that change with the direction in silicon include the Young's Modulus (a measure of the strength of the material), the mobility of electrons (or holes), the etch rate, and the oxidation rate. For example, the Young's modulus of silicon is 1.3 e12 dynes/cm2 in the (100) crystal plane, 1.7 e12 dynes/cm2 in the (110) crystal plane, and 1.9 e12 dynes/cm2 in the (111) crystal plane. As another example, the mobility of electrons in the (100) crystal plane is known to be greater than in the (110) crystal plane of silicon, resulting in a current drivability in the (100) direction that is approximately 15 percent (%) greater than the current drivability in the (110) direction.
Substrates of varying layers having different crystal orientations can have a high degree of lattice mismatch. “Lattice mismatch” is the percentage difference between atom spacings along a plane of the same crystallographic orientation as the interface plane in the respective bulk phases of the two materials. In certain instances, lattice mismatch can lead to device degradation in semiconductor devices. Thus, an interfacial layer is generally required to accommodate lattice mismatch between layers of different crystal orientations in multi-layered substrates.
In some wafer bonding techniques resulting in substrates of different crystal orientations, hydrophilic bonding can be used. For example, a thick layer of oxide is formed on a donor wafer. The oxide layer can be in the range from 300 Angstroms (Å) to 3000 Angstroms. The wafer surfaces are typically terminated in hydrophilic hydroxyl groups, such as Si—OH groups. The donor wafer is subsequently bonded to the handle wafer by contacting the treated surfaces of the respective wafers to each other with heat applied thereto. Prior to bonding, one of the wafers is implanted with hydrogen to establish the ‘breakline’ for predetermined thickness of the top silicon portion of the bonded wafer. When the bonded wafer is treated with heat, the implanted hydrogen line breaks and thus a wafer with an imbedded thick oxide layer is formed. Thus, the result is a hybrid crystal orientation substrate with a thick oxide layer in between the respective crystal orientation layers.
Imbedded thick oxide bonding produces a wafer with imbedded isolation (buried oxide, or BOX) which has different device characteristics than standard bulk silicon. This requires completely different circuit design. In addition, the interfacial oxide layer acts as an insulator and the resulting hybrid substrate has silicon-on-insulator (SOI) characteristics.
In some wafer bonding techniques resulting in substrates of different crystal orientations, hydrophobic bonding can be used. The process requires high temperatures and expensive equipment. For example, a handle wafer can be treated with hydrofluoric acid. A donor wafer can be implanted with hydrogen ions resulting in an implanted wafer. Before contacting the treated surfaces, the handle wafer must be subjected to a pre-anneal process to obtain a surface free of hydrogen (H) and hydroxyl (OH) groups. In some applications, the pre-anneal process is conducted at 650 degrees Celsius (° C.). Similar to the handle wafer, the implanted wafer must be processed by a krypton-fluoride excimer laser to obtain a surface substantially free of H and OH groups. Thereafter, the handle wafer and the implanted wafer can be contacted for wafer bonding. The result is a substrate which may include layers of different crystal orientations formed by high temperature and laser treatment.